A lot of studies for increasing the luminescent efficiency of organic electroluminescence elements are being made. Up to now, various kinds of efforts have been made to increase the luminescent efficiency by newly developing and combining an electron transport material, a hole transport material, a light-emitting materials and others that constitute an organic electroluminescence element. Above all, a phosphorescent material can use the excited triplet state thereof and has a high quantum efficiency, and is therefore specifically noted as a light-emitting material. However, the phosphorescent material has a common challenge in that it readily undergoes triplet-triplet deactivation, and therefore, for preventing such deactivation, various investigation have been made (see NPL 1).
A compound having a triphenylphosphine oxide structure has a high lowest excited triplet energy level as compared with conventional carbazole compounds, and is therefore specifically noted as a host material for light-emitting materials. For example, PTL 1 says that selective use of a compound having a lowest excited triplet energy level of not lower than 2.65 eV could improve a luminescent efficiency, describing an organic electroluminescence element that has a configuration where a layer of a compound having the following structure and doped with a light-emitting material is laminated on a layer of 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (NPB).

NPL 2 describes an organic electroluminescence element that has a configuration where a hole transport layer formed of 4,4′,4″-tris(carbazol-9-yl)triphenylamine (TCTA) is laminated on a light-emitting layer of a compound having the following triphenylphosphine oxide structure and doped with iridium (III) bis(4,6-difluorophenyl)pyridinato-N,C2)picolinate (FIrpic).

NPL 3 describes an organic electroluminescence element that has a configuration where a light-emitting layer of 9-(3-(9H-carbazol-9-yl)phenyl)-3-(dibromophenylphosphoryl)-9H-carbazole (mCPPO1) doped with bis(3,5-difluoro-4-cyanophenyl)pyridine) iridium picolinate (FCONIrpic) is laminated on a layer formed of a compound having the following triphenylphosphine oxide structure. This reference says that the organic electroluminescence element having the configuration can attain a high luminescent efficiency.

NPL 4 describes an organic electroluminescence element that has a configuration where a light-emitting layer of a compound having the following triphenylphosphine oxide structure and doped with iridium(III) bis(4,6-difluorophenyl)pyridinato-N,C2)picolinate (FIrpic) is laminated with a hole transport layer formed of 4,4′,4″-tris(carbazol-9-yl)triphenylamine (TCTA). This reference says that the organic electroluminescence element having the configuration can attain a high luminescent efficiency.

NPL 5 describes a compound having the following triphenylphosphine oxide structure. This reference further describes an organic electroluminescence element that uses the compound having such a triphenylphosphine oxide structure as the electron transport material therein.
